The present exemplary embodiments relate to phosphors for the conversion of radiation emitted by a light source. They find particular application in conjunction with converting LED-generated ultraviolet (UV), violet or blue radiation into white light for general illumination purposes. It should be appreciated, however, that the invention is also applicable to the conversion of radiation in other applications, such as Hg-based fluorescent lamps, mercury-free gas discharge lamps, metal halide lamps, UV, violet and/or blue lasers, as well as other white light sources for different applications.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicating lights or in other applications where colored light is desired. The color of light produced by an LED is dependent on the type of semiconductor material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). To form the LEDs, layers of the alloys are typically deposited epitaxially on a substrate, such as silicon carbide or sapphire, and may be doped with a variety of n and p type dopants to improve properties, such as light emission efficiency. With reference to the GaN-based. LEDs, light is generally emitted in the UV and/or blue range of the electromagnetic spectrum. Until quite recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Manufacturers are continuously looking for new colored phosphors for use in such LEDs to produce custom colors and higher luminosity.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs are based on blue emitting GaInN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complementary color, e.g. a yellow-green emission. The total of the light from the phosphor and the LED chip provides a color point with corresponding color coordinates (x and y on the 1931 CIE chromaticity diagram) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
The CRI is commonly defined as a mean value for 8 standard color samples (R1-8), usually referred to as the General Color Rendering Index and abbreviated as Ra, although 14 standard color samples are specified internationally and one can calculate a broader CRI (R 1-14) as their mean value. In particular, the R9 value, measuring the color rendering for the strong red, is very important for a range of applications, especially of medical nature. As used herein, “CRI” is used to refer to any of the above general, mean, or special values unless otherwise specified.
Incandescent lamps are known to generate an excessive amount of light in the yellow region of the spectrum, leading to “washing out” of the colors of objects illuminated by them. In response to this, methods have been developed to filter out part of the yellow light through the use of the rare earth element neodymium in the glass envelope. Examples of such lighting systems include the line of REVEAL® light bulbs produced by GE.
A salient feature of the spectrum of REVEAL® bulbs is the deep depression in the yellow region, resulting from the filtration by the neodymium glass, as shown in FIG. 5 (solid line). This yellow depression does not extend to zero spectral intensity, but only to about 15% of the highest intensity of the spectrum between 400 and 700 nm. As a result, the REVEAL® bulbs have a deficiency in the yellow part of the spectrum versus the reference illuminant (shown as a dashed line in FIG. 5). This deficiency leads to increased red-green color contrast when objects are viewed under this lamp, in comparison to a regular incandescent bulb. This has been found to be very appealing to customers.
This filtration significantly reduces the light generated by the lamp. In incandescent lamps, this reduction is tolerable due to the sufficient brightness of such lamps. It would be desirable to develop new lighting systems with similar characteristics as the REVEAL® bulbs. However, a significant lumen loss in new lights (LED lights in particular) is generally unacceptable. Thus, it would be desirable to develop white lamps with similarly enhanced color contrast but without losing any light through filtering.
In addition, it would be desirable to obtain sources with enhanced color contrast and having correlated color temperatures higher than those achievable by neodymium glass-filtered incandescent sources, e.g. significantly higher than 3000K. The invention described herein overcomes this restriction.